The present application relates to construction equipment, such as cranes. In particular, the present application relates to a construction machines that include an electronically controlled hydraulic circuit. In the instance of cranes, the hydraulic circuit operates to control the rotation or swing of an upper portion of the crane relative to a lower portion.
Previous cranes typically used a hydraulic circuit that employed what are known as spool valves to control the hydraulic motor that turned the upper portion of the crane relative to the lower portion of the crane. These spool valve based systems posed several challenges, however.
First, an off-the-shelf spool valve typically was not available for a given application. Therefore a specialized, highly customized, and application-specific spool valve typically was required. Understandably, these specialized spool valves typically were quite costly. Further, by nature of the spool and manufacturing tolerances, the valves often had imprecise flow rates and suffered from leakage across the valve at what were ostensibly no flow conditions.
In addition, the manufacturing imperfections increased the possibility of uneven movements when changing from rotating in one direction to the other. Such uneven movements typically were the outward manifestation of pressure spikes within the hydraulic circuit, pressure spikes that pose an elevated risk of damaging the hydraulic circuit.
For example, one effort to resolve this issue involved using open center spool valves, which led to improved and softer counter-slewing (i.e., changing the direction of rotation without coming to a complete stop). The open center spool valve, however, led to inconsistent starts.
Alternatively, closed center spool valves resulted in smoother and more consistent initiation of a rotation. Unfortunately, the trade-off was less satisfactory performance during counter-slewing, including abrupt shifts.
Regardless of the specific type of spool valve, collectively these issues lead to crane-specific hydraulic circuits that require individual calibration of the control systems. These issues typically prevented the ability to achieve perfectly symmetric flow in each portion of the hydraulic circuit that controls the each direction of rotation of the hydraulic motor. In other words, the hydraulic circuit might behave differently when slewing or rotating clockwise than it would when slewing or rotating counter-clockwise. While some of this might result might be accounted for in the (often crane-specific) calibration of the hydraulic circuit, it nonetheless poses a challenge for the crane operator who must retain awareness about the crane-specific issues with its performance.
It is therefore desirable to provide a crane with a hydraulic circuit to control the swing or rotation of an upper portion relative to a lower portion of the crane. The hydraulic circuit should provide for easier calibration, consistent operation amongst different cranes, smoother and more consistent starts to rotation and counter-rotation/counter-slewing.